The social media craze is aimed at raising awareness and money for the neurodegenerative disease ALS, or motor neurone disease.

Mr Darling thanked McAvoy and tweeted a link to www.gordonsfightback.com , an awareness-raising site set up by Better Together director of research Gordon Aikman.

Mr Aikman publicly revealed he is dying of the disease in a powerful article in the Scotland on Sunday newspaper in June.

In his video, X-Men star McAvoy said: "As it is the year of the Scottish referendum on independence, I'd like to nominate our political champions Alistair Darling and Alex Salmond to do their very worst."

As a bucket of icy water is poured over his head, he adds: "Alex, Alistair, you've got 24 hours."

McAvoy said he was nominated for the challenge by Chelsea manager Jose Mourinho.

Former US president George Bush, actress Gwyneth Paltrow and footballer David Beckham are among a host of well-known faces that have taken the plunge.

McAvoy, from Glasgow, has previously said he is "anti-political" when it comes to independence.